Electrostatic writing tubes



ELECTROSTATIC WRITING TUBES Casper W. Barnes, Jr., Palo Alto, Calif., assignor to B. Dick Company Application May 13, 1957, Serial No. 658,791

4 Claims. (Cl. 178'6.6)

This invention relates to electrostatic writing tubes and, more particularly, to improvements therein.

An electrostatic writing tube consists of a linear array of wires which are embedded in and pass through a nonconducting dielectric medium. This medium is supported in a faceplate for a vacuum tube which contains an Means are provided for modulating the electron beam,

or turning it off and on, as it is deflected across the wires. A ground plane is placed behind the dielectric writing medium so that the medium passes between the ends of the writing wires and the ground plane.

The writing wires are all insulated from each other and are not connected to any other part of the circuit.

As the electron beam strikes a particular wire, the po-.

tential of this wire will begin to approach the potential United States Patent of the cathode of the electron gun, which is negative with respect to the ground plane. After a certain critical amount of charge is placed on this wire by the beam, it reaches a critical potential with respect tothe ground plane and an electric discharge will occur between the end of the wire and the writing medium. This charge will deposit a spot of charge on the writing medium.

If the electron-beam current is properly modulated as'it scans along the writing wires, and as the paper is being pulled past the ends of the wires, it is possible to place any desired charge pattern on the surface of the paper. The charge pattern on the paperrcan then be developed, using conventional electrostatic developing techniques.

Thus, in the electrostatic writing tube described, the writing electron beam performs the functionsof charging up the wires to the critical potential and then supplying charge to the writing medium as the discharge occurs. The current required in the writing beam to perform these functions may be such as to require a relatively large modulating signal on the electron-gun grid and, further, may serve to limit the minimum electronbeam diameter obtainable. Any limit on minimum electron-beam diameter obtainable will, in turn, limit the resolution obtainable in the charge pattern on the paper.

An object of this invention is to provide a novel electrostatic writing tube wherein the current required of the writing beam is less than has been required heretofore.

Another object of the present invention is to provide an improved electrostatic writing tube requiring lower modulating voltages than could be used heretofore.

Yet another object of the present invention is to provide a novel and useful electrostatic writing tube which provides better resolution and greater sensitivity than previously known electrostatic writing tubes.

These and other objects of the invention are achieved in an electrostatic writing tube which has a second electron gun from which there may be derived the charge the holding beam, by itself, does not cause any discharge between the writing wires and the dielectric writing medium. Thus, the current which is required from the writing-electron beam will be very much less than that required where the writing beam has to provide all of the current. This decrease in writing-beam current does permit smaller beam diameters to be obtained; thereby, the resolution of the tube can be increased and smaller modulating voltages may be employed on the writing beam control grid. 7

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawing, which is a view in section of an embodiment of the invention.

Referring now to the drawing, there will be seen a view in section of an embodiment of the invention. This comprises an evacuated envelope 10 having at one end a target 12, which includes a conductive metal plate -14 having a slot at the center thereof wherein there is positioned a plurality of separate writing wires 16 within a dielectric medium 18, such as glass. The separately spaced writing wires 16 have one of their ends terminating at the outside of the tube face to form a column of writing points. A ground plane, or conductor, 20' is spaced opposite the column of writing points. A dielectrio writing material, such as paper 22, is moved transverse to the column of writing points between the ground plane and the writing points.

The embodiment of the invention has two electron guns 30, 32, one of which is the writing gun and the other is the holding gun. The writing gun 30 includes a cathode 34, which is biased negative from an operating potential source 36. The positive terminals of the operating potential source are coupled to the ground plane. A control grid 38 is used to control the intensity of the emission from the cathode 34. Video signals for controlling the beam emission are received from a suitable video signal source 40. After the usual accelerating electrodes (not shown), a focusing electrode 42 focuses the electronsfrom the cathode into an electron beam of a width-suitable to impingeupon a wire. The focus 'coil is followed by a deflection coil 44, which receives deflection signals from a sweep-signal source 46. Deflection, in this instance, is in one direction only, since all that is required is for the beam to sweep the column of wires.

As thus far described, the electrostatic printing tube does not distinguish over the art as it is known previous to this invention. The video signals modulate the beam intensity as the sweep signals move the beam across the wires, so that the combination of the motion of the paper transverse to the column of wires, the motion of electron beam along the wires, and the video-signal modulation of the electron beam result in an electrostatic charge pattern being deposited on the paper having the appearance dictated by motion and the video signals. Well-known electrostatic charge-developing techniques wherein a powder is made to adhere only to charged areas can be used to produce a visible image on the paper. 7

The tube, as thus far described, must provide all of the charging and discharging current for the wires and thus, to obtain any degree of speed of operation, it is required that the electron beam supply a substantial current. This requires a large electron beam, and, in addition, requires that signals for the modulation thereof have .largeramplitudes. In accordance with this.inv.ention,,the :second Iholding gun32 includes a cathode 54, followed by the .same grid structure as was found in the .case .of the writing gun 30. The cathode is biased from thesame potential source 36, but at a less negative potential than the .cathode 34. The beam which emanates from the cathode 54, however, is not focused as a result of which it will fall upon all the wires in the target simultaneously. Thus, these wires are all charged up toward the potential on thecathode 54. As a result, the only function required .o f the writing beam is that of triggering the discharge between the writing wires and the paper. With the potential difference between the holding-beam cathode and the ground plane being made less than the critical potential for writing, then the holding beam by itself doesnot cause any discharges between the writing wires of the paper.

With the structure shown, a very small current in the writing beam can trigger-a discharge between the writing wires and the paper. This writing-beam current is very much less than that required in the electrostatic printing tube, which only has a single electron gun. This decrease .in writing-beam current thus permits smaller beam diameters to be obtained, since less current must beprovided from these, beams. Thus, the resolution of the writing tube is greater and the requirements for the modulating video voltages on the writing-gun grid are :made far less. Thereby, the sensitivity of the tube is improved.

The operation of the two-gun tube for writing is identical with a single-gun tube. The holding beam is turned .on all the time and is never modulated.

.Accordingly, there has been described and shown here- .inabove a novel, useful, and simple and improved .elec- "trostatic writing tube with greater sensitivity and per- ;mitting greater writing resolution.

'I claim:

.1. In an electrostatic writing tube of the type having a target comprising an array of a plurality of wires terminating in a column of points at a writing surface on the .outside of said tube, a ground plane opposite said column of points, means to produce a beam of electrons, means for repetitively sweeping said beam of electrons across said array of wires and means for selectively controlling .said electron beam as it sweeps across said array of wires to raise the potential of selected ones of said wires until they discharge to said ground plane, the improvement in :said tube comprising means to continually bias all said -wires at a potential below the potential at which they discharge to said ground plane including an auxiliary electron gun in said tube, and means to direct electrons Irom said auxiliary electron gun to all said wires.

amass 2. In an electrostatic writing tube of the type having a target comprising an array of a plurality of wires terminating in a column of points at a writing surface on the outside of said tube, and electron beam means to elevate the potential of selected ones of said wires until a discharge occurs from their points to an adjacent ground plane, the improvement comprising means to generate electrons at a potential below the discharge potential of said wires and means to continuously direct electrons from said means to generate at said plurality of wires.

3. An improved electrostatic writing tube comprising an evacuated envelope, at target in one end of said envelope including a plurality of spaced insulatingly supported wires with one of their ends providing a column of writing points on the outside of said tube, a ground plane positioned outside said tube adjacent said column of writing points, a first and a second electron gun in said envelope opposite said target means for focusing electrons from said first gun into an electron beam, means .for defiectingsaidelectron beam across said plurality of wires,.n1eans for biasing said first electron gun to a potential at which a discharge to said ground plane occurs from :a wire upon which said beam falls, means for directing electrons .fromsaid second electron gun upon all :said wires, and means for biasing said second electron gun to :a potential below that of said first electron gun.

4. improved electrostatic writing tube comprising an evacuated envelope, a target in one end of said envelope including a plurality of spaced insulatingly supported wires with one of their ends providing a column of writing points on the outside of said tube, a ground plane positioned outside said tube adjacent said column of writing points, a first cathode in said envelope for producing electrons, -a central grid for controlling the intensity of said electrons, means for focusing said electrons :fromsaidfirst cathode into a beam, a source of .operating potential, means for applying a first potential fromisaid source to said first cathode having an intensity sufficient to 'cause .a discharge from a wire point upon which said beam vfalls to said ground plane, means to ,deflect said beam across said plurality of wires, a sec- :ond :c-athode, means-to direct electrons from said second cathode to all said plurality of wires, and means for applying-a second potentialfrom said source to said second cathode which is-less than said wire discharge potential.

References Cited in the file of this patent UNITED STATES PATENTS 2,276,359 VonArdenne Mar. 17, 1942 2,291,476 Kernkamp July 28, 1942 2,657,377 Gray: Oct. 27, 1953 12,771,336 ,MacGrifi Nov. 20, 1956 32,777,745 .McNaneyv Jan. 15, 1957 

